Field of the Invention
The present invention relates to a process for producing lower alkyl alcohols, such as ethanol, from cellulosic biomass using primarily anaerobic microorganisms under thermodynamically favorable conditions therefor; and to anaerobic microorganisms that produce lower alkyl alcohols, such as ethanol, from cellulosic biomass and which microorganisms are tolerant to the lower alkyl alcohols produced.
Description of the Background
The U.S. patent application Ser. No. 12/000,856, filed on Dec. 18, 2007; and U.S. Provisional Application No. 60/870,441, filed on Dec. 18, 2006, are incorporated herein by reference in their entirety.
Currently most ethanol fuel produced in the U.S. is made from corn grain, a feed stuff. Further, even if all the corn grain produced in the U.S. were converted to ethanol, it would only supply about 15% of our current transportation fuel needs in replacing that amount of petroleum usage. Thus, there is a pressing need to produce fuel ethanol from cellulosic plant fiber instead. If ethanol could be inexpensively produced from sources other than corn grain, waste biomass like leaves; paper; manure; and wood or wood byproducts, for example, further inroads into replacing petroleum usage could be made. Cellulosic biomass can be grown on marginal land and in greater yields than grain crops and, thus, offers the promise of high yield based on input. Eventually, it is likely that the U.S. could use up to a billion tons of such biomass per year.
Presently, however, the available technologies for producing so-called cellulosic ethanol are prohibitively expensive and, thus, industrially unfeasible. Moreover, competition for available corn grain between use in ethanol fuel and in producing corn-based food stuffs has led to a large rise in the price of such food stuffs for consumers.
There are three processes generally available for the production of ethanol from plant fiber, also called plant cell wall, which contains cellulose, hemicelluloses, pectin, and lignin. One process is called physical conversion where biomass is heated to high temperatures, such as 650° F., in the absence of oxygen. The biomass is degraded to carbon monoxide (CO) and hydrogen (H2), and subsequently these gases are converted to ethanol by a catalytic or microbial process. Unfortunately, this process involves substantial facility costs and is not considered cost effective for commercial use.
A second approach is biochemical conversion which entails boiling the biomass in caustic acids or other chemicals to unravel the cellulose and hemicelluloses. The residue is neutralized and conditioned and subjected to cellulolytic enzymes to release sugars. The glucose released is fermented by yeast to ethanol, and the 5-carbon sugars are separated and converted to ethanol by a different microorganism.
A third approach to producing cellulosic ethanol would be to use living microorganisms that could digest cellulose, hemicelluloses and pectins with conversion to ethanol. This approach might, in theory, be the least expensive approach because it would not require the use of harsh chemicals or high temperatures and would use fewer process steps with fermentation than would the two approaches described above. Although this approach has been considered for over twenty years, it has, unfortunately, not been used as it is only feasible if there is a microorganism or mixed culture of microorganisms that can readily digest cellulose and hemicellulose, and which, preferably, convert a significant part of the carbohydrate to ethanol. Further, the preferred microorganisms must also be tolerant to relatively high ethanol concentrations, i.e., at least about 3-4% by volume, and preferably in excess of 5% by volume, so that they may be used to digest considerable carbohydrate to produce ethanol at high enough concentration to decrease the cost of distillation.
Currently, microorganisms for effecting this process in a cost effective manner are unknown. Microorganisms are known which can readily digest cellulosic biomass to produce organic acids, but not ethanol. Further, although microorganisms are known which produce trace amounts of ethanol along with acetic acid, these microorganisms are intolerant to ethanol concentrations exceeding a small amount of media volume of ethanol which renders separation of ethanol from aqueous media cost prohibitive. Ethanol tolerant microorganisms not only grow in relatively high ethanol concentrations, however, it would be even more advantageous for such microorganisms to be able to grow from the energy they capture by producing more ethanol. Moreover, the preferred conditions for fiber digestion (e.g. neutral pH) differ from the preferred conditions for ethanol production (e.g. acidic pH) within existing microorganisms.
The conventional wisdom among scientists is that there is little reason to continue pursuing efforts at isolating naturally-occurring microorganisms for cellulosic ethanol production. In fact, the US Department of Energy (DOE) Roadmap for biofuel production (DOE, 2006) also emphasizes similar points and reinforces this conventional wisdom. The grant programs by the DOE and US Department of Agriculture (USDA) encourage modem metabolic engineering and discourage attempts to find and isolate suitable naturally-occurring microorganisms for the production of ethanol.
Thus, a need exists for a plausible method for producing ethanol, as well as other lower alkyl alcohols, from cellulosic biomass using microorganisms and fermentative digestion, thus avoiding the use of costly process steps to produce the ethanol or other lower alkyl alcohols. Yet, the possibility of finding naturally-occurring microorganisms for this task has been dismissed for the most part.